Previous experience with clinical bone marrow transplantation has shown that normal marrow is capable of providing life-long systemic metabolic correction for patients with MPS diseases; however, translation of this principle to ex vivo hematopoietic stem cell gene therapy has been limited by: (a) the inability of MLV-based retroviruses to integrate into non-dividing cells; (b) high-titer vector preparations to achieve a substantial MOI; and (c) immunologic selection against transgene-expressing cells. However, recent development of third-generation lentiviral vectors suggest an approach that may obviate these problems with broad implications for many applications of ex vivo hematopoietic stem cell gene therapy. The overall objective of this project is to accomplish the requisite preclinical studies for ex vivo hematopoietic stem cell gene therapy for mucopolysaccharidosis type I (i.e., Hurler syndrome and variants). Toward this objective, Specific Aim 1 will be to construct HIV based lentiviral gene transfer systems that will be useful for treatment of human neuropathic conditions represented by the MPS diseases, and particularly targeted at transduction in human hematopoietic stem cells. Specific Aim 2 will be to evaluate the potential for intravenous administration as a means of in vivo lentiviral bone marrow stem cell transduction and metabolic correction, as well as the potential for in vivo selection of transduced cells. Follow-up evaluations will study the response to genetically-modified hematopoietic stem cells, specifically, determining the level of recombinant MPS enzyme activity achieved, the duration of expression, and clinical response. Toward an initial clinical trial, Specific Aim 3 will study vectors for expression of the normal IDUA human enzyme delivered by ex vivo lentiviral transduction of G-CSF mobilized CD34+ human peripheral blood progenitor cells after engraftment into NOD/SCID mice. In phases of this study, more complex bicistronic message vectors will be studied for the ability to enhance IDUA gene expression by means of in vivo selection of transduced cells based on antifolate drug resistance). Aliquots of G-CSF mobilized CD34+ human peripheral blood progenitor cells will be treated with IDUA vector, or IDUA-DHFR vector, and studied for (a) transduction of CFC; (b) transduction of LTCIC; and (c) transduction of marrow-repopulating cells in NOD/SCID mice. These studies are directly aimed at developing a clinical trial of ex vivo hematopoietic stem cell gene therapy for the model disease MPS I. However, the studies will have much greater significance for the broader application of lentiviral vector systems and especially for application in hematopoietic stem cell gene therapy.